1. Field of the Invention
Aspects of the present invention relate to antenna beamforming in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system. More particularly, aspects of the present invention relate to transformation codebook antenna beamforming in an OFDM wireless communication system.
2. Description of the Related Art
Transmit beamforming may be performed in a wireless communication system in either a closed-loop or open-loop manner. Open-loop transmit beamforming is typically well suited for use in a Time Division Duplexing (TDD) system, which does not require channel information feedback. By not feeding back the channel information, less overhead is used. However, open-loop transmit beamforming has a disadvantage in that phase calibration should be constantly performed in order to compensate for the phase difference between transmission and reception of Radio Frequency (RF) chains among multiple transmit antennas. The process of phase calibration is generally costly, and sensitive to the radio channel environment.
In contrast, closed-loop transmit beamforming typically does not require phase calibration if channel sounding pilot signals (e.g., common pilot signals, midambles, and preambles) are also used for data demodulation. If channel sounding signals are not used for data demodulation, a dedicated pilot signal would be needed for data demodulation. In this case, a phase calibration would still be needed for closed-loop transmit beamforming in a Frequency Division Duplexing (FDD) system. It is noted that closed-loop transmit beamforming requires the feedback of channel information to the transmitter. By feeding back the channel information, more overhead is used. In addition, closed-loop transmit beamforming is also sensitive to feedback channel error due to feedback delay or fast channel variation. Typically, an FDD system employs the closed-loop transmit beamforming scheme. It is noted that a closed-loop scheme can also be applied to TDD systems.
A practical closed-loop transmit beamforming scheme is typically based on a codebook design. The closed-loop codebook-based transmit beamforming can be used for a scenario where a base station forms a transmit antenna beam toward a single user (or a mobile station) or simultaneously toward multiple users (or multiple mobile stations) at a certain time and at a certain frequency. A codebook is a set of pre-determined antenna beams that are known to mobile stations. It has been known that a codebook based pre-coding Multiple-Input and Multiple-Output (MIMO) can provide significant spectral efficiency gain in the downlink of a system implementing closed-loop MIMO. In the Institute of Electrical and Electronics Engineers (IEEE) 802.16e and 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standards, a 4 Transmit (TX) antenna limited feedback based closed-loop MIMO configuration is supported. Nowadays, in the IEEE 802.16m and 3GPP LTE Advanced standards, in order to provide peak spectral efficiency, an 8 TX antenna configuration is proposed as a prominent closed-loop MIMO downlink system.
To eliminate the need of phase calibration in a case where channel sounding signals or common pilot signals (e.g., common pilot signals, midambles, and preambles) are not used for data demodulation, a closed-loop transformed codebook based transmit beamforming has been proposed. The transformation codebook utilizes the channel correlation information for two folds, namely to modify a fixed codebook (or a base codebook) to enhance antenna beamforming performance in highly correlated wireless channels and to remove the need of antenna phase calibration for multiple transmit antenna systems. Typically, the channel correlation information described above is based on second order statistics of a channel correlation matrix and can be fed back from a receiver to a transmitter when closed-loop transmit beamforming. Typically, the channel correlation information changes very slowly, which is similar to a long term channel effect, such as the shadowing and path loss. Thus the feedback overhead and computation complexity from correlation information is very small.
Therefore, a need exists for techniques for transformation codebook antenna beamforming in a wireless communication system.